1. Field of the Invention
The invention is generally related to devices and materials used to remove mercury and other heavy metals, particulate matter, as well as other solid, liquid and vapor components which may have an adverse environmental impact, from a hot gas stream generated from power plants, industrial exhaust systems, reaction vessels, waste incinerators, or other sources.
2. Description of the Prior Art
To prevent adverse environmental consequences, the effluent gaseous stream from power plants, industrial exhaust systems, reaction vessels, waste incinerators, and other sources, must be cleaned-up to remove harmful and/or toxic constituents. The clean-up operation is often referred to as "scrubbing", and devices used to accomplish the clean-up are referred to as "scrubbers".
Through the Clean Air Act of 1990, the Environmental Protection Agency has instituted legislation to control emissions of 189 air toxins, including mercury, which result from various industries. A considerable amount of research has been focussed on removing the major pollutants, SO.sub.x, NO.sub.x, and primary particulates, from gaseous streams; however, trace metals such as mercury, lead, zinc, nickel, molybdenum, cadmium, gallium, beryllium, antimony, copper, and vanadium, and other constituents such as chlorine, fluorine, and arsenic, are produced by the combustion of fossil fuels such as coal and can pose significant health hazards. Conventional flue gas clean-up systems such as cyclones, electrostatic precipitators, and filter bags can effectively remove large fly ash particles, but are generally ineffective in filtering trace metals which exist in the fine particulate or vapor phase. With particular reference to mercury produced during coal combustion, only a small portion of flue gas mercury will condense on particulate and be removed through scrubber action by passing the effluent through fabric filters or the like as part of the flue gas desulfurization process. 80-90% of the mercury will remain in the vapor phase and will pass through the sulfur scrubber and will be emitted from exhaust stack into the ambient environment. Chlorine and fluorine released during fossil fuel combustion or other reaction processes also remain in the vapor phase and are discharged directly into the environment without being removed from the exhaust stream. In addition, lead, antimony, copper, vanadium, zinc, arsenic, beryllium, cadmium, gallium, molybdenum, nickel, etc., are byproducts of fossil fuel (coal) combustion and will form sub-micron particles through nucleation processes. Sub-micron particles are too small to be removed by conventional industrial exhaust gas scrubbers. The sub-micron particles pose a significant health hazard because they are respirable (e.g., most asthma treating drugs are administered as sub-micron particles).
Hence, there is a need for a scrubber that can effectively remove vapor phase trace metals such as mercury and sub-micron particles from the exhaust streams generated from power plants, industrial exhaust systems, reaction vessels, waste incinerators, or other sources.
U.S. Pat. No. 4,695,447 to Shultz discloses the use of molten aluminum in the treatment of hazardous wastes. In operation, mercury oxides and sulfides are reduced to elemental mercury which is captured via condensation.
U.S. Pat. No. 4,814,152 to Yan discloses a chemisorbent composition and process from removing elemental mercury from a gas. The chemisorbent composition contains a solid support, elemental sulfur, and at least one metal which catalyzes the reaction 2 Hg+S.sub.2 .fwdarw.2 HgS.
U.S. Pat. No. 4,902,665 to Elfline discloses removing heavy metals from waste water by adding a water insoluble carboxylated cellulose transition metal oxide to the water to precipitate the heavy metal onto the cellulose.
U.S. Pat. No. 4,564,509 to Shealy et al. shows bubbling reactant gases through a ternary melt to remove oxygen, water vapor, and other oxygen bearing gas species.
U.S. Pat. No. 4,094,777 to Sugier et al. discloses the use of a copper sulfide containing solid mass to remove mercury from a gas or a liquid.
U.S. Pat. No. 4,093,702 to Merkl discloses a method of removing gaseous pollutants such as NO, NO.sub.2, SO.sub.2, SO.sub.3 and halogens, from a stream of gas by contacting the gas stream with an aqueous medium which contains an activated form of aluminum.
U.S. Pat. No. 3,193,987 to Manes et al. shows impregnating activated carbon with metals which will amalgamate with mercury.
Yeh et al., Report PERC/RI-76/5, Trace Contaminants in Coal, Ed. S. Torrey, Noyes Data Corporation, 1978, presented investigation results using high efficiency filter bags to remove mercury vapor and trace metal particulate from coal combustion gases. Although relatively high mercury filtration efficiencies were achieved under certain conditions, in general, the success of this method depends upon high levels of carbon in the fly ash. High levels of carbon are indicative of incomplete combustion. In addition, high filter loadings were required, which increases the pressure drop across the filter, and consequently, increases the power requirement of the forced-draft or induced draft fan in a conventional power plant. Finally, a lower flue gas temperature was shown to be advantageous with the filter bags, although there is a practical minimum operational temperature of 270.degree. F., which is the dew point of acid gases.
The use of activated carbon as an adsorbent is used in many industrial clean-up systems. Activated carbon has been investigated for mercury removal in coal combustion and municipal waste disposal environments. Geiser et al., "Control of Mercury from MSW Combustors by Spray Dryer Absorption Systems in Activated Carbon Injection, Municipal Waste Combustion Conference, Williamsburg, Va., Mar. 30-Apr. 2, 1993, reported that using activated carbon in flue gas clean-up was inconsistent and complicated.
Friedlander, Report PB-273 236, Trace Contaminants in Coal, Ed. S. Torrey, Noyes Data Corporation, 1978, discussed the use of electrostatic precipitators in exhaust gas clean-up. It was reported that electro-static precipitators are often ineffective at filtering sub-micron (0.1-1 .mu.m) particulate from exhaust gas streams, and suggested that increases in either the collection plate area or power input would be required to significantly enhance the filtration capability of sub-micron particles.